Salts have been used for a variety of reasons to aid in the separation of acrylic acid and water.
The predominate use of salts has been to allow simple phase separation that results in a first phase containing 80 wt.% or more acrylic acid with water, and a second phase containing approximately 10 wt.% acrylic acid, water and the salt. U.S. Pat. No. 2,922,815 discloses that in the prior art, such drying agents as calcium chloride and sodium sulfate have been used to increase the concentration of acrylic acid-water systems from 35-40 wt.% acrylic acid to 50-60 wt.% acrylic acid. Further, nickel chloride or nickel bromide can be used to achieve concentrations of up to 80%.
U.S. Pat. No. 3,663,375, although directed to the separation of methacrylic acid from water, discloses a process suitable for the separation of acrylic acid wherein sodium sulfate is first mixed with the acid, followed by a two-phase separation. Both phases are then passed to separate distillation columns wherein the acid is removed from the rich phase, and a stream of water, salt and some acid removed from the dilute phase. However, use of sodium sulfate or sulfuric acid with acrylic acid has shown little or no phase separation. Thus, while the processing steps may be similar, different acids would have to be utilized.
U.S. application Ser. No. 860,516 discloses the use of aluminum chloride to achieve a 95 wt.% concentration of acrylic acid in the rich phase from an aqueous solution containing 50 wt.% acrylic acid.
Salts have also been shown to have other effects on acrylic acid-water solutions. U.S. Pat. No. 3,846,488 discloses that certain salts at low concentrations reduce the amount of time necessary for the separation of the phases to occur. U.S. application Ser. No. 860,937 discloses certain salts that eliminate the eutectic point of an aqueous acrylic acid solution, thereby allowing fractional crystallization.
It is also known that certain solvents, such as ethyl acetate, can extract acrylic acid from the aqueous solution using liquid-liquid separation (see U.S. Pat. No. 3,344,178). U.S. Pat. No. 3,846,488 notes that this liquid extraction efficiency can be increased by the addition of salts that affect the liquid-liquid equilibrium.
While the above salts have been effective in concentrating acrylic acid in the rich phase, there still remains a dilute acrylic acid phase, usually containing 20 wt.% or less acrylic acid, saturated with salt. Because of the relative volatilities between acrylic acid and water, fractional distillation of this stream is difficult and substantial losses of acrylic acid are realized in the separation. This is because acrylic acid is always more concentrated in the liquid than in the vapor in equilibrium with it.
It has been discovered that certain salts, independent of any solvents, can substantially reverse the relative volatility of acrylic acid and water, thus allowing a simple method of concentrating dilute acrylic acid solutions.